In general, a drum type washing machine performs a washing on laundry articles by using a gravitational potential energy produced when it rotates a cylindrical drum repeatedly in a water tub to lift them to an upper portion of the drum and then allow them to drop.
Such the drum type washing machine is advantageous in that damages caused to the laundry articles by their entangling and an amount of washing water can be reduced when compared with pulsator type and agitator type washing machines.
Such the drum type washing machine, as shown in FIG. 1, includes a drum 120, for providing a washing space, rotatably installed in a water tub 110 horizontally installed on an upper portion of a base 130, and a detergent supply container 140, for supplying a washing detergent including a washing assisting agent, such as a fabric softening agent, installed above the water tub 110. Further, between an upper portion of a housing (not shown) and the water tub 10, a plurality of tensile coil springs (not shown) are installed to absorb vibrations and impacts produced by a rotation and stop of the drum 120 or falling of the laundry articles during the washing and dewatering process.
Further, between a lower portion of the housing and the water tub 110, oil dampers 160 are installed. So the vibration and the impacts can be reduced more effectively by the damping action of the dampers 160 along with the elastic force of the coil springs.
The drum type washing machine 200 with the configuration described above usually has a drying function and allows a drying process to be performed after completing the washing and dewatering process. Therefore, the drum type washing machine 200 further includes a drying device in an upper and rear portion thereof, which is provided with a chamber 170 having a heater therein, a blower 175 and a condensation duct 180. The drying device circulates a drying air repeatedly into the inside of the drum 120 and condenses moisture contained in the drying air, thus drying the laundry articles.
Meanwhile, on a rear surface of the water tub 110 of the drum type washing machine 200, a motor for rotating the drum 120 is provided. Such the driving motor is usually constructed by an electric induction motor in which a rotational force is generated by an interaction between a rotating magnetic field generated in a stator and an inductive magnetic field generated in the rotor. The electric induction motor can be designed in various ways, i.e., it can be designed as a three-phase induction motor, a three-phase winding type induction motor and so forth as well as a single-phase induction motor. And it has a capability to maintain a constant rotational speed and a long lifetime.
The electric induction motor used as the driving motor of the drum type washing machine 200 basically includes a stator generating an induced magnetism by receiving a power from outside via a winding coil wound around a core; and a rotator rotating a rotation shaft of the drum 120 combined therewith through a shaft coupler by the induced magnetism generated by the stator.
In the electric induction motor with the above-described configuration, an electric current is induced to a secondary winding by an electromagnetic induction of a primary winding which is connected to a power supply, and a rotary power is obtained by an interaction between the current induced at the secondary winding and a rotating magnetic field. Such an electric induction motor can be classified into an inner rotor type or an outer rotor type depending on relative locations of the stator and the rotor.
In the inner rotor type motor, since the rotor rotates in the stator, a diameter of the rotor is restricted. Therefore, it has a disadvantage that a torque per unit volume is low. And an inner space thereof cannot be used.
Recently, an outer rotor type induction motor having a rotor installed outside a stator has wide applications, because it is capable of increasing a torque at a same volume, and, by using the outer rotor type motor, it is possible to use the inner space of the stator for another purpose.
The rotor of the outer rotor type induction motor is, as shown in FIG. 2, manufactured to be shaped into a hollow cylinder according to a drawing method by a press.
The rotor 10 is provided with a plurality of radiation holes 1 formed in a bottom portion thereof continuously in a radial direction. The radiation holes 1 are for discharging heat produced during the operation of the motor to the outside by circulating air through the radiation holes.
However, since a contact area of the rotor 10 with air is not sufficient, the heat produced during the operation of the motor cannot be dissipated effectively only with the radiation holes 1. Therefore, the efficiency of the motor declines sharply.
To overcome such a problem, recently the rotation holes 1 are formed by cutting a portion of the bottom of the rotor corresponding to the rotation hole 1 along its boundary except for one side, and then by bending the cut portion about the non-cut side outwards to form a cooling fin 2.
The cooling fin 2 can increase the contact area of the rotor 10 with air, thus improving the cooling effect. However, a width of the radiation hole 1 should be less than a predetermined value to prevent safety accidents such as insertion of fingers therein.
In particular, due to characteristics of the manufacture method, the shape of the cooling fin 2 is limited to a rectangle. Therefore, there are several drawbacks; heat exchange by the contact with air cannot be maximized, and outside air cannot be introduced into the inside of the rotor 10 through the radiation holes 1 during forward and backward rotation of the rotor 10, thus deteriorating the cooling effect.
Accordingly, when the drum type washing machine employing the conventional rotor 10 described above is operated for a long period of time, its efficiency will decline sharply. As a result, the washing cannot be performed appropriately.